This invention relates to a method of improving the operation and efficiency of a steam generating furnace adapted to burn high moisture content fuel comprising a novel method for pre-drying the fuel prior to its entry into the combustion chamber.
Waste wood, as a by-product of the paper mill industry, has long been burned in furnaces to generate steam. Hogged or waste wood generally has a very high moisture content, being on the order of 50-70% water. In the past, in order to maintain stable burning conditions of waste wood, a secondary fuel such as oil or natural gas has been necessary. In recent years, as these fuels have become scarcer and more expensive, ways have been sought for burning wet waste wood with a minimum use of secondary fuels. Pre-drying the wood helps in this regard, causing it to burn faster and hotter with greater stability, higher efficiency and higher specific combustion rates.
High moisture content fuels, of course, are not limited to wood. Examples of other types of such fuels include bagasse, peat, sludges, garbage, etc. As used herein the term "wet fuel" or "high moisture content fuel" is intended to mean and include any type of such fuel suitable for burning in a furnace.
One present means of drying high moisture content fuel prior to burning it is to introduce it into the upper portion of the combustion chamber of a furnace, so that some of the moisture is removed as it falls downwardly through the ascending hot products of combustion onto a grate at the bottom. Such an arrangement is shown in Glaeser, U.S. Pat. No. 2,483,728, and in Wood, U.S. Pat. No. 1,427,045. Another arrangement is disclosed in Lis, et al., U.S. Pat. No. 4,213,405 in which fuel fed onto an inclined grate is pre-dried as it descends and is then discharged onto a second grate.
A major disadvantage of the Glaeser and Wood method is that unburned fuel and pollutants are entrained in the gases leaving the furnace rather than being retained and burned on or above the grate. A disadvantage of the method disclosed by Lis, et al. is the large grate area and special furnace arches that result in higher first costs and maintenance.
Fluidized bed furnaces, as described in Bryers, U.S. Pat. No. 3,893,426, are another means which have been used to burn high moisture content fuels. Bryers provides two combustion chambers in which hot gas flow is upward in both. Solid fuel is introduced into only one of them. A characteristic of fluidized bed furnaces is that they must be operated at temperatures (1200.degree. F.-1800.degree. F.) significantly lower than those used in traditional combustion furnaces (2200.degree. F.-2400.degree. F.). The present invention is not applicable to fluidized bed furnaces of the type disclosed by Bryers.
It is, accordingly, an object of the invention to provide in a high moisture content fuel burning steam generating furnace of the traditional combustion type a novel and relatively inexpensive method for predrying the wet fuel so as to increase the stability and efficiency of burning of the fuel while reducing the generation and release to the atmosphere of particulate and gaseous pollutants.